Detailed investigation of polymineral ceramic designed for Nuclear High Level Wastes (HLW) fixation was carried out. The samples of ceramic were obtained in 'RADON' enterprise using a high frequency 'cold crucible' technique. It consists in melting initial oxides mixture under 1400 Centigrade degrees and crystallisation of the melt obtained at lower temperature. Bulk chemical composition of the ceramic was chosen close to conventional SYNROC-C formulation produced by solid state hot pressing method and was as follows, in weight %: TiO2-58.5, CaO-11.8, ZrO2-11.4, BaO-7.8 and A12O3-10.5. The initial composition was doped with stable isotopes of Cs, Sr, Ni, Fe, Ce, Nd and Mo in quantitaties of 1 to 2% of each element.
Mineral and chemical composition research of the melted SYNROC were performed by optical, x-ray structural, electron microprobe and electron microscopy methods. It was determined that material consists of major zirconolite with ideal
stoichiometry CaZrTi2O7, hollandite (BaA12Ti6O16), perovskite (CaTiO3), minor rutile (TiO2), small hibonite (CaAl12O19) and alkaly earths molybdate phases. Electron diffraction patterns of the phases are consistent with monoclinic system of zirconolite, orthorhombic system to perovskite, and tetragonal system to hollandite, rutile and molybdate. Lattice parameters of the minerals were calculated. Electron microprobe analysis showed wide variations in phases composition which could be explained in terms of isomorphous substitutions in minerals crystal structure. Study of elements partitioning among minerals indicates that hollandite is a main host mineral for Cs, perovskite and hollandite for Sr, zirconolite and hollandite for Fe and Ni, zirconolite and perovskite for rare earth elements. Comparison of the phases composition of melted SYNROC with those of hot pressing ceramics and their natural mineral analogs were performed too.
Matrix form of nuclear wastes is a main artificial barrier preventing hazardous radionuclides leaching and migrating from their underground disposal sites into biosphere. So searching for resistant materials with high ability to incorporate the components of nuclear wastes is extremely actual. A study of about two hundred published and original chemical analyses of zirconolites sampled from a number of alkaline and carbonatite intrusives of the world, including West Europe, South America, Antarctica, and Russia as well as Lunar zirconolites was carried out. Variations in zirconolite composition are explained considering crystal structure of the mineral and empiric Goldschmidt rules for elements' isomorphic exchange. Comparison of published data on geochemical properties of natural zirconolites and synthetic samples obtained by hot pressing (ANSTO, Australia) with original results for the mineral produced by melt crystallisation method ('RADON', Russia) was performed. It was established that zirconolite is the most appropriate matrix mineral for fixation of actinides, some fission products (Zr, Nb and rare earth elements) and radioactive contaminants (Fe, Co and Ni).